1. Field of the Invention
The field of the present invention is laser communications.
2. Background of the Invention
High quality video and audio signals and high bandwidth data signals (called “broadband” signals) are becoming increasingly desirable in today's digital world. A significant challenge is getting high bandwidth communications to end users, or reaching the so-called “last mile” market segment. Most U.S. metro centers are serviced by multiple providers over SONET fiber optic rings, with fiber to certain major buildings. Many, if not most, buildings, are not on fiber rings, however, and laying fiber can be time consuming and prohibitively expensive. In some instances, it may be practically impossible to obtain property rights-of-way to provide a high-bandwidth connection to the desired location.
While wireless radio frequency (RF) systems can provide data rates of 155 Mbps, there is limited spectral bandwidth available, communication licenses are generally required, the possibility for mutual interference exists, and the requisite equipment is expensive. Extending to higher data rates is difficult for RF frequencies with good atmospheric propagation characteristics.
Atmospheric laser communication provides a potential alternative for wireless point-to-point communications of high bandwidth signals. For instance, laser transceivers are capable of sending high bandwidth signals through the atmosphere. However, commercially available laser systems capable of transmitting high bandwidth signals across distances longer than a small city block are prohibitively large and extremely expensive. Moreover, several challenges must be overcome to facilitate high bandwidth laser communications over significant distances. One consideration is ensuring reliable communications despite varying atmospheric conditions. Since conditions such as fog in particular are difficult for low power laser beams to penetrate, ensuring uninterrupted atmospheric laser communications requires the use of high power lasers. A second design consideration is preventing high power laser beams used in an atmospheric laser system from causing eye or tissue damage if received by people. At short wavelengths, non-eyesafe power levels can permanently damage the eye before the victim becomes aware, because the retina has no pain sensors
Further complicating the use of atmospheric lasers is a phenomenon called scintillation that causes the random fading of signals transmitted through the atmosphere. It is understood that the atmosphere is not homogeneous, in that the index of refraction of air is not constant due to wind or turbulence. The transmission of a beam of light through the atmosphere is subject to these variations in the index of refraction such that the beam may be momentarily deflected from a straight path. With such deflection, an observer of the beam perceives the source to be flickering. Such flickering is highly disruptive to data transmission. A solution may be found in aperture averaging, by increasing the size of the apertures of the receiving unit. The intensity of the source can, to a certain extent, mitigate losses in transmission where the sensitivity of the receiver is not correspondingly decreased. Often, however, physical and practical limitations detract from such solutions.
To significantly overcome the effect of scintillation, spatial diversity transmitters have been constructed which employ multiple diode lasers arranged to produce displaced parallel beams. As these beams diverge, they overlap one another. A receiver displaced from the transmitter thus receives uncorrelated light at the receiver when aligned with the beams. As it is unlikely that all beams will be simultaneously diverted, the receiver is able to receive uninterrupted data from at least some of the plurality of transmitters. It has been found that the normalized standard deviation of the intensity at the receiver is reduced by the square root of the number of transmitting elements when properly separated. Reference is made to W. M. Bruno, R. Mangual, & R. F. Zampolin, Diode Laser Spacial Diversity Transmitter, pp. 187–194, SPIE vol. 1044, Optomechanical Design of Laser Transmitters and Receivers (1989), the disclosure of which is incorporated herein by reference.
One structural application of the very principles presented in the foregoing publication is found in U.S. Pat. No. 5,777,768, the disclosure of which is also incorporated herein by reference. Transceivers using spaced multiple laser transmitters are used for two-way communication.
Another example of laser transceivers used for communications purposes may be found in application Ser. No. 09/434,913, filed Nov. 5, 1999, for a Portable Laser Transceiver, the disclosure of which is further incorporated herein by reference. The portable laser transceiver disclosed therein is capable of transmitting near-broadcast quality video, audio, and Ethernet signals.
For broadband fiber optic applications, a number of pre-fabricated integrated circuits are available for driving lasers at data rates of 1 gigabit per second (Gbps) or more. These laser drivers are used to drive the now-common fiber optic networks. These integrated circuits, however, are inadequate for high power lasers used in atmospheric laser communications, as they typically provide drive current capability of only 50 to 75 mA. Such low drive current is insufficient to overcome the effects of atmospheric scintillation at distances beyond of approximately the length of a laboratory.
When a high power laser is used, one method that has been used to achieve the high drive current needed to overcome atmospheric scintillation effects is the use of a RF Bias-Tee. This method typically uses a 50 ohm bias tee, thus coupling the broadband signal into a 50 ohm load—typically consisting of a 47 ohm matching resistor in series with a 3 ohm laser diode—to achieve a broadband match. The RF bias-tee approach, however, is not practical for high drive currents because the majority of the output power is wasted in the matching resistor. For example, a 700 mA drive current typically results in 5.8 watts of power dissipation in the 50 ohm bias tee.
A high current 4:1 broadband RF transformer may be used with a bias-tee approach to double the output drive current and transform the 50 ohm into a 12.5 ohm source, as seen by the load. However, this alternative approach still requires a 9 ohm resistor to match the source to a 3 ohm laser diode. Thus, the majority of the drive power is still wasted in the matching resistor. A transformer with a higher ratio could theoretically solve the lost power problem, but high ratio transformers capable of handling currents in excess of 200 mA and having a broadband response of up to 1 Ghz are not available.
U.S. Pat. No. 5,521,933 discloses a method of positioning the laser diode remotely from the driver circuit to reduce the effect on the laser diode of heat generated by the driver circuit. This method still uses a matching resistor, located remotely from the laser diode, which again causes power loss in the output drive current.
Regardless of the driving frequency, when driving a laser diode at high power, it is frequently desirable to use a thermoelectric cooler (TEC) to maintain the temperature stability of the laser diode. In many applications, the temperature stability of the laser diode may be important to maintain the output signal of the laser diode within a specified set of parameters. The cooling function of a TEC is controlled by a TEC controller circuit. Typical implementations of TEC controllers are either pulse-width-modulated (PWM) or proportional controllers. PWM controllers are undesirable for use in communication systems with sensitive receivers because PWM controllers tend to generate unwanted noise. Proportional controllers such as the proportional-integral-differential (PID) type are therefore commonly used in such communication systems.
PID controllers, however, tend to dissipate the most heat when maximum cooling at the laser diode is required. The heat dissipation occurs because PID controllers function as a current source, having a compliance voltage that is significantly less than the supply voltage. For example, a PID controller operating off a 5V supply at its maximum rated current output typically has a useable compliance voltage of about 3V. The difference between the supply voltage and the useable compliance voltage tends to be dissipated in the controller as heat. Thus, in the most demanding cooling conditions such as hot weather, a PID controller tends to generate even more heat.
A need, therefore, exists for small and efficient, yet powerful laser transceivers that are capable of transmitting and receiving high power and high bandwidth signals across distances greater than a single city block. A need also exists for a means to efficiently cool high powered laser transmitters used in such transceivers.